An introduction to ONIX metadata
https://storyfit.com/blog/onix-metadata-how-to
Another introduction to ONIX metadata
https://bookmachine.org/2015/01/22/non-technical-beginners-guide-onix-books/
And another introduction to ONIX
https://www.consonance.app/blog/a-non-technical-beginners-guide-to-onix-for-books/
Some information about what the Publishing status code number means
https://onix-codelists.io/codelist/64
ONIX information from the standards maintainer
https://www.editeur.org/83/Overview/
ONIX file naming
https://www.editeur.org/202/FTP-Filenaming/
More ONIX file naming (pdf)
https://www.editeur.org/files/FTP%20filenaming/080519-ftp-filenaming-issue-4a.pdf
Even more ONIX file naming (pdf)
https://www.editeur.org/files/FTP%20filenaming/file_naming_convention_v03.pdf
Centraal Boekhuis ONIX API information (Dutch only)
https://wiki.cb.nl/display/BERDEFINTERN/ONIX+3.0+bericht+Titelbank
PHP code for parsing ONIX XML
https://github.com/chrisribal/onix-parser
More PHP code for parsing ONIX XML
https://github.com/ansas/php-onix
Perl code for parsing / importing ONIX XML
https://github.com/nbezzala/Onix